(1) Field of the Invention
The present invention relates to a charging device for use in an image forming apparatus which forms images with a toner (developer) based on the electrophotographic technique, in particular, relating to a charging device for charging the photosensitive member on which toner images are formed.
(2) Description of the Prior Art
Conventionally, a copier using an electrophotographic system (generally called as a reversal development system) in which the photosensitive drum charged with the same polarity as the toner is exposed to light to form a static latent image and then this static latent image is developed with the toner forming a toner image, includes a charging device for charging the photosensitive drum at a uniform level before exposure, an exposure device, a developing unit and the like, all being arranged around the photosensitive drum. The system also includes a transfer device, a cleaning device and an erasing device.
The photosensitive drum surface is subjected to a series of image forming steps effected by these devices. FIG. 1 shows the processing areas of the different devices on the photosensitive drum A surface. Here, when the lengths of the charging area, exposure area (=image area), developing area, transfer area, cleaning area and erasing area are respectively represented by L1, L2, L3, L4, L5 and L6, the following relations should hold:
L2 less than L1 or L3 or L4xe2x80x83xe2x80x83(1) 
L1 or L3 or L4 less than L5 less than L6xe2x80x83xe2x80x83(2) 
The relationship (1) provides reliable development of the static latent image formed by charging and exposure and reliable transfer of the developed image. Actually, the surface potential becomes unstable or uneven at the boundaries of the charging area, so that it is necessary to provide a predetermined charging margin for the image area. Further, since development becomes unstable or uneven at the boundaries of the developing area, it is necessary to provide a predetermined developing margin for the image area. Since the transfer voltage at the boundaries of the transfer area is unstable and uneven, it is necessary to provide a predetermined transfer margin for the image area. The relationship (2) provides reliable cleaning and erasing of the photosensitive drum surface. That is, the cleaning area needs a cleaning margin with respect to the maximum length among L1 to L4, and the erasing area needs a predetermined erasing margin with respect to the cleaning area. It should be noted that the magnitudes of L1, L3 and L4 are not specified.
However, in such a copier, since the non-image area which is outside the charging area or which is not electrified on the photosensitive drum, is low in potential because of no charge, the toner is liable to adhere to this area during the developing process. Therefore, this configuration has the problem in that the toner adhering to this area will pollute the print paper and the copier interior and also promote toner consumption.
To solve the above problems, Japanese Utility Model Application Laid-Open Hei 3 No.125358 discloses a copier with a scorotron type charging device, which has a grid for adjusting or varying the amount of charge and making the charge distribution uniform in the image area on the photosensitive drum. In this disclosure, it is specified that the grid should have a length greater than the range of the image area and both ends of the grid residing within the charging area but outside the image area should be curved closer to the photosensitive drum side. In comparison with the configuration where the grid is not curved closer to the photosensitive drum side, this disclosure is able to prevent the toner from adhering to sites outside the image area where the absolute value of the surface potential is lower than the absolute value of the developing potential (the surface potential of the charging area has the same polarity as the developing potential because of the reversal developing system) and hence is able to prevent black strip defects on the paper.
FIG. 2 shows a scorotron type charging device. A discharge element 1 for generating charged particles for applying static charge over the photosensitive drum surface is attached to an insulative electrode holder 2 and housed by a charger case 3. Arranged in front of discharge element 1 is a grid 4 that regulates the passage of the particle flow from discharge element 1 so as to make the charge distribution uniform. This grid 4 is engaged at its both ends with claws 6 projected from a holder 5 so that it is supported whilst being tensioned by the holder 5. Designated at 7 is an electrode. Here, a charging device with no grid 4 is called as a corotron type. The scorotron type is superior to the corotron type in its capability to improve the uniformity of the surface potential of the photosensitive drum.
However, since the above configuration in which the grid is curved close to the photosensitive drum side deprives the grid of its flatness across the image area, there occurs another problem in that the uniformity of the potential distribution across the image area is degraded. Further, since the surface potential on the photosensitive drum in the non-image area and outside the charging area is lower in absolute value than the developing potential, the toner will adhere to the areas outside the charging area but within the developing area. This not only consumes unnecessary toner but also increases the waste toner to be collected from the photosensitive drum, leading to reducing the life of the waste toner container. Further, even in a configuration where a cleaning area wider than the range of the developing area is provided as stated above, when the toner is scraped from the photosensitive drum by the cleaning blade, part of the toner will move along the cleaning blade edge toward the sides of the photosensitive drum. In this way, the toner adhering to the areas outside the image area and being left untransferred will cause extra stress on the seal elements arranged at both side ends of the cleaning blade, damaging the sealing performance in an early stage.
In order to solve the above problems, Japanese Patent Application Laid-Open Hei 4 No.172379 discloses a copier having a corotron type charging device. This publication discloses a technique for prevention against the toner adherence as above by setting the charging area at a length greater than the developing area and the image area on the photosensitive drum surface within which the static latent image is formed. Though it is possible to prevent the above-described toner adherence properly, this configuration has no grid and hence is unable to enhance the uniformity of the potential distribution across the image area. Thus, it is not only impossible to expect high quality images but also there is a risk of image defects occurring due to charging unevenness.
Further, there is another problem as follows. To simplify manufacturing, some photosensitive drums are produced so that the conductive substrate of the photosensitive drum is coated with a photosensitive layer, not covering the substrate edges but leaving some uncoated parts a certain distance from the edges. Alternatively, in order to remove the pooling of the application liquid at the edges of the substrate, excessive application of liquid at the edges is wiped off after the application of the photosensitive layer. However, this wiping may cause the electric insulation at the edges to lower compared to the portion from which the photosensitive layer has not been wiped off. When a grid is, in a simple manner, arranged across such a photosensitive drum, the low-insulative portions, that is, the naked substrate edges or the substrate edges from which the photosensitive layer has been wiped off, come close to the grid, so that leakage discharge may occur between them, possibly damaging the photosensitive layer. The same problem also occurs when flanges of the photosensitive drum are formed of a conductive resin or the like in order to ground the substrate.
In view of the above, it is therefore an object of the present invention to provide a charging device capable of charging the image area uniformly while preventing toner adherence to the non-image area. It is another object of the present invention to provide a scorotron-type charging device which can prevent leakage discharge from the grid to the photosensitive member.
The means for solving the problems according to the present invention is attained by the combination of scorotron and corotron charging modes. The merits of both modes are made use of to uniformly charge the image area whilst preventing toner adherence to the non-image area. That is, the image area on the photosensitive member surface is charged by the scorotron mode while the non-image area is charged by the corotron mode.
To achieve this, the grid should be configured of a regulating portion opposing the image area and at least one non-regulating portion opposing the non-image area. The non-regulating portion should at least oppose the developing area outside the image area, instead of opposing the entire non-image area. In this arrangement, for the image area on the photosensitive member surface, the flow and passage of the particles from the discharge element is regulated by a predetermined voltage so as to be able to supply a stable, uniform flow of particles. The non-regulating portion allows the flow of particles from the discharge element to pass therethrough so as to raise the potential of the developing area outside the image area to equal to or a higher level than the developing potential. Thus, the non-image area can be set at a potential equal to or higher the developing voltage, thus preventing toner adherence in this area.
Further, the arrangement of the non-regulating portions of the grid a predetermined distance away from the photosensitive member makes it possible to prevent the grid set at the predetermined voltage from causing leakage discharge to the low-insulative portions at the photosensitive member edges. In this case, the non-regulating portions can be configured, in a simple manner, by providing openings in the grid. Further, tensioning the grid in the direction away from the photosensitive member prevents the degradation of the flatness of the grid, allowing for stable charging. This manipulation enables the grid to be arranged in proximity to the photosensitive member.